I think this story is filled with irony, but you'll need to decide which type(s) of irony you wish to identify and write about.
** verbal irony -- for example, when a character says one thing but means another
** dramatic irony -- several possibilities, including when "the character expects the opposite of what the reader knows that fate holds in store"
** situational irony -- "in which accidental events occur that seem oddly appropriate"
See the lit_terms link below and scroll down to Irony to read about various types of irony in detail.
Irony is a contrast between appearance and actuality. In this story, the innocent young boy thinks he's in love with Mangan's sister. He sees himself as a religious hero and equates the young girl to the Virgin Mary. His feelings for her at the beginnning are very strong. "But my body was like a harp and her words and gestures were like fingers running upon the wires." He confuses his crush on her with his religious teachings and goes to Araby, a bazaar sponsored by the church, to buy her a present when the young girl says she can't go. He tells her he'll buy her something if he goes to the bazaar.
The young narrator expects the bazaar to be magical, a place that reflects his feelings for the girl and his enthusiasm for his religion. He has an "epiphany" after he arrives, a moment of insight where he understands the actuality of his feelings for the young girl. Once he understands his feelings for her, he then sees Araby for what it is, just a place to buy trinkets. He feels angry at himself and probably is disappointed, but this is part of his transformation from innocence to a more mature understanding of what it's all about. He expects one thing before he gets to Araby, but his "epiphany" allows him to see the reality of what it really is.
Though irony is not the most dominant element of "Araby," it (irony) does play a part in the story. One place you can see irony is in the tension between appearance and reality, and between appearance and vision. The story starts with the line " NORTH RICHMOND STREET being blind…" Well, it is "blind" in the sense of being a dead end, but also in the sense of people not seeing things clearly there. The houses stare at one another, even though one of them is empty; the narrator seems to encounter only a symbolic façade at the end of the story. Likewise, there is irony in the distance between the anticipated wonder of the bazaar and the narrator's disappointment at the end of the story, where he finds himself " as a creature driven and derided by vanity."